On Thursday, firefighters navigated steep, rugged terrain in an effort to contain a rapidly spreading fire in the hilly area northwest of Los Angeles. Propelled by strong winds and dry conditions, the fire swept through multiple homes, injuring at least two people and leading to the evacuation of more than 10,000 residents.

Named the Mountain fire, the blaze ignited in Ventura County on Wednesday morning and expanded to over 20,000 acres by Thursday afternoon, endangering 3,500 structures, according to fire officials. With containment efforts at zero percent, teams were set to survey the affected areas to assess the number of homes damaged or destroyed.

Firefighters on the ground, along with helicopters, concentrated their efforts around the Santa Paula and Somis communities. Over 800 firefighters were already deployed on Thursday, with additional support expected. Several schools in the area were closed, and officials advised residents to avoid the vicinity.

“The fire is in a challenging location — it’s rugged, steep terrain that only our most skilled firefighters can access,” explained Capt. Trevor Johnson of the Ventura County Fire Department. “We’ll do everything we can.”

Dozens of evacuees spent the night at Padre Serra Parish, a Catholic church in Camarillo, a town about 50 miles northwest of Los Angeles known for its retiree population and scenic wedding venues.

Linda Fefferman and her husband, Dennis Gottlieb, were among those sheltering at the church after evacuating the home they rented from a friend. They had a narrow escape as a tree toppled, blocking the road as they left the neighborhood.

“We thought we were trapped,” recalled Ms. Fefferman, 74. “Luckily, a neighbor had a chainsaw.”

Their home didn’t survive. The couple lost two pet birds, extensive family genealogy records, and supplies for Mr. Gottlieb’s pest control business. “He’s got his truck, but everything else is gone,” Ms. Fefferman said.

With no renter’s insurance, they plan to stay at the shelter for now. They have no nearby family or alternative options. “Eventually, I’ll break down, I’m sure,” Ms. Fefferman added.

Along West Highland Drive, where the Feffermans lived, scenes of ruin were interspersed with houses that appeared untouched.

Some properties were reduced to heaps of charred wood, with only a chimney or a burnt beam left standing, while at least eight homes remained unscathed, their lawns still green.

Firefighters continued to monitor remaining hotspots amid smoky, ash-filled air.

“The fire there is in a tricky place — it’s rugged, steep ground that only our finest firefighters can even access,” said Capt. Trevor Johnson of the Ventura County Fire Department. “We’re going to work as best as we can.”

Dozens of evacuees spent the night at Padre Serra Parish, a Catholic church in the town of Camarillo. The community, which is about 50 miles northwest of Los Angeles, is home to many retirees and scenic wedding venues.

Wind speeds, which had been as high as 80 miles per hour, eased to around 35 m.p.h. on Thursday, providing some relief, said National Weather Service meteorologist Rich Thompson. Conditions were expected to improve by Friday.

The area threatened by the fire is dotted with nurseries, farms, and horse ranches. While wildfires are familiar to residents, they were taken aback by the rapid spread of this blaze.

Governor Gavin Newsom announced that FEMA would provide swift financial aid for firefighting efforts and urged residents to heed evacuation warnings.

Two people had been hospitalized with fire-related injuries on Wednesday, according to Ventura County Sheriff Jim Fryhoff, though their conditions were unknown. The fire’s cause remains under investigation.

For residents, the fire struck unexpectedly. Ethan Bates, 26, left his Camarillo home early Wednesday for a round of golf 30 miles away. When he returned around 10:30 a.m., the neighborhood was engulfed in smoke.

“It was like a nightmare — a massive black cloud hanging in the air,” Mr. Bates said. He grabbed his passport and his golden retriever, Butter, and left with his mother.

Meanwhile, his mother’s boyfriend stayed back, using a generator to power a water pump and hose down the house and nearby vegetation. He sent photos of the lawn and palm trees ablaze.

By Thursday, Mr. Bates had returned to find his home still standing, though the yard was scorched, and his neighbor’s tractor was completely destroyed.

“I truly believe if her boyfriend hadn’t stayed, the house would have been gone,” he said.